9 Phenytoin (Dilantin) Side Effects and How to Manage Them
Key takeaways:
Phenytoin (Dilantin, Phenytek) is an antiepileptic medication that treats seizures. It requires routine lab testing to prevent low or excessive medication levels in your body.
High amounts of phenytoin can cause side effects such as unpredictable eye movements, coordination problems, and slurred speech. Phenytoin side effects generally require attention from a healthcare provider. Thankfully, they’re often manageable with a dose adjustment.
Serious side effects like confusion and skin rashes can also occur with phenytoin. While rare, they warrant more immediate medical attention.
Table of contents
Phenytoin (Dilantin, Phenytek) is an antiepileptic medication that for decades has been FDA approved to treat seizures. It can treat one-time seizures and epilepsy; epilepsy is a chronic health condition where seizures happen repeatedly or frequently.
Phenytoin works by preventing nerves in your brain from sending abnormal signals. This can be effective for controlling seizures, but it can also cause unwanted side effects. You’re more likely to experience side effects with higher levels of phenytoin.
The ideal level of phenytoin in your body is typically 10 mg/L to 20 mg/L. If your phenytoin level jumps too high, side effects become more of an issue. For this reason, people who take phenytoin need routine lab tests to make sure their dosage is effective, but not toxic. Thankfully, most side effects are temporary and reversible.
Below, we’ll discuss nine notable phenytoin side effects. Keep in mind that this isn’t a complete list of side effects. And if you develop any bothersome side effects while taking phenytoin, speak with your healthcare provider as soon as you can.
1. Unpredictable eye movements
Unpredictable eye movements, also known as nystagmus, are a common side effect of phenytoin. Your eyes may tend to drift horizontally — even if your phenytoin level is where it should be. If your level is too high, your eyes may move rapidly in different directions.
It’s hard to manage these eye changes on your own. So you should contact your healthcare provider if you’re having strange eye movements after starting phenytoin. They may need to check your phenytoin level and adjust your dose. This side effect is usually mild and should go away when your phenytoin level is lowered.
2. Coordination problems
Coordination problems, also known as ataxia, are another common phenytoin side effect. You may notice that you feel off balance, walk unsteadily, or have difficulty with your fine motor skills.
If you’re experiencing new or worsening coordination problems, let your healthcare provider know. They will likely need to adjust your dosage. Coordination problems typically appear when your phenytoin level exceeds 30 mg/L.
To reduce your risk of falls and injury, do your best to take safety precautions. For example, you can remove potential hazards, like slippery shower mats, in your home. Cutting down on how much you drive is another good idea. You can also try exercises such as tai chi or yoga to enhance your balance and coordination.
3. Slurred speech
You may develop unclear or slower speech when taking phenytoin. These are a sign that your phenytoin level is too high. Like with coordination problems, slurred speech tends to appear when phenytoin levels exceed 30 mg/L.
Thankfully, these changes are reversible for most people. If you find that you’re having trouble communicating your thoughts and ideas, you should contact your healthcare provider right away. They may adjust your dosage or recommend a different medication for you.
However, if speech changes are significant or long-lasting, your provider may refer you to a speech therapist. Speech therapy can help you recover communication skills more effectively.
4. Nausea and vomiting
Phenytoin can sometimes make you feel nauseated. You’re more likely to experience stomach problems, such as nausea or vomiting, when your phenytoin level is above 30 mg/L.
If you experience stomach problems when taking phenytoin, let your provider know. They may want to adjust your dose. But there are also some management steps you can take on your own.
You can take phenytoin with or without food. But taking it with a meal can help minimize nausea. At the same time, try to avoid foods that trigger nausea, such as spicy or acidic foods.
Good to know: Be consistent with how you take phenytoin. For instance, if you start taking phenytoin with breakfast, you should take it with breakfast every day. This will help your phenytoin level remain stable.
5. Tiredness and confusion
Excessive fatigue (tiredness) and sudden confusion are possible symptoms of phenytoin toxicity — excessively high phenytoin levels. These side effects mostly occur with a phenytoin level over 40 mg/L. This is different from fatigue you may normally feel after a long day of work or a poor night of sleep.
If you experience these symptoms, go to the nearest emergency room right away. These symptoms are a possible sign that your phenytoin level is very high. In severe cases of phenytoin toxicity, this could lead to more seizures or a coma.
Your healthcare provider will likely measure your phenytoin blood level to confirm the toxicity. In this case, they’ll keep an eye on you until you feel better. These symptoms should subside as your phenytoin level drops to normal.
6. Skin rashes
Although rare, serious skin rashes have been reported with phenytoin. These skin rashes are the result of allergic reactions known as Stevens-Johnson syndrome (SJS) or toxic epidermal necrolysis (TEN).
Skin rashes are more likely to appear within the first month of taking phenytoin. But they can happen at any time. SJS and TEN can be life-threatening if they’re not addressed quickly, so let your provider know right away if a skin rash appears on your body. This is important even if the rash appears to be mild. Your provider will likely stop phenytoin and switch you to another anti-seizure medication.
Some people are at an increased risk of skin rashes due to their genetics. If you or someone in your family has a history of allergic reactions or skin irritations, your healthcare provider may want to discuss genetic testing with you.
7. Swollen gums
It’s less common, but phenytoin can cause an overgrowth of the gum tissue around your teeth. This side effect is more likely for people who take phenytoin for a long time or at higher doses.
Since your gums may be more sensitive, try to maintain good oral hygiene when taking phenytoin. Continue to brush and floss regularly to minimize the overgrowth of gum tissue. Your healthcare provider may also recommend more frequent visits to your dentist.
8. Withdrawal problems
It’s not recommended to stop taking phenytoin suddenly. Your dose of phenytoin should be lowered over a period of time, usually several months. This helps prevent withdrawal symptoms. Abruptly stopping phenytoin can cause more and potentially worse seizures.
It’s important to work with your healthcare provider to slowly decrease your phenytoin dosage. You should receive a structured dosing plan called a taper. This dosing plan reduces the risk of withdrawal seizures.
9. Depression, including thoughts of suicide
As with many other anti-seizure medications, phenytoin carries a possible risk of depression and suicidal thoughts or behavior. Changes in your mood can begin as early as 1 week into starting phenytoin.
Let your healthcare provider know right away if you experience any mood changes or thoughts of self harm while taking phenytoin. They may make changes to your dosage and offer other resources to help you manage your symptoms. You can also Call the National Suicide Prevention Lifeline at 988 or text HOME to 741-741 to reach the Crisis Text Line.
When to contact a healthcare provider about phenytoin side effects
If you’re experiencing any new or worsening side effects, let your provider know. Based on your current phenytoin level, they may adjust your dosage or offer other management tips. But don’t adjust your dosage or stop taking phenytoin on your own — talk to your provider first. Any abrupt changes to your phenytoin dosage can increase your risk of side effects.
Certain medications and health conditions can increase your risk of side effects from phenytoin. Be sure to give your provider your full medical history. This will help them check for drug interactions and make any necessary adjustments.
The bottom line
Although phenytoin is effective for treating seizures, it has some risks. Phenytoin side effects may include involuntary eye movements, coordination problems, and slurred speech. In most cases, side effects are related to a high level of phenytoin and can be managed with dose adjustments. Some side effects, such as skin rashes, are rare, but they require prompt medical treatment.
Why trust our experts?
InformedHealth.org [internet]. (2023). What is speech therapy? National Library of Medicine, National Center for Biotechnology Information.
Iorga, A., et al. (2023). Phenytoin toxicity. StatPearls.
Kilpatrick, C. J. (2004). Withdrawal of antiepileptic drugs in seizure-free adults. Australian Prescriber.
Remedy Repack Inc. (2023). Phenytoin sodium- phenytoin sodium capsule, extended release [package insert].
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